1. Field of the Invention
The invention relates to a heat accumulation control device which controls printing operation of a line-type thermoelectric printer in response to heat accumulation of a thermal head.
2. Prior Art
The line-type thermoelectric printer, conventionally known, has a thermal head consisting of one line of head elements so that printing is performed by transferring ink onto a paper by each line. In general, it is demanded that a printing speed of the printer should be made faster. So, in order to achieve high-speed printing by the line-type thermoelectric printer, it is demanded to shorten an interval of time required between two lines of printing.
Therefore, the line-type thermoelectric printer should inevitably perform printing of a current line before heat, produced by printing of a previous line, is completely radiated from the thermal head. That is, heat must be accumulated in the thermal head by printing of respective lines. The thermal head is designed to transfer ink onto a paper in a density (or gradation) which responds to temperature of the thermal head. So, heat accumulation will cause a relatively large deviation of density (or nonuniformity of printing) between a first line and a last line printed on the paper.
According to one method of suppressing such a deviation of density, an amount of residual heat (i.e., a residual-heat value) of the thermal head is predicted based on previous input data, so that current input data are corrected based on a prediction value; and then, printing is performed using the current input data corrected. Now, one example of a construction of the line-type thermoelectric printer employing the above method will be described with reference to FIG. 3.
In FIG. 3, a numeral 1 designates a data-input section which is provided to sequentially input lines of data from a computer. Each data is designated by `D.sub.jk `. Herein, `j` is a variable representing a number assigned to a specific head element of the thermal head. If a total number of head elements, contained in the thermal head, is `n`, the variable j is an integer which meets inequality of 1.ltoreq.j.ltoreq.n. In addition, `k` is a variable representing a line number designating a specific line to be printed on the paper. If a total number of lines which form one page of printing is `L`, the variable k is an integer which meets inequality of 1.ltoreq.k.ltoreq.L. Incidentally, a set of data, corresponding to one line of printing regarding a line number k, as a whole is designated by `D.sub.k `.
A numeral 2 designates an allocation section which uses the data D.sub.jk, sequentially inputted by the data-input section 1, to allocate them to `m` groups by using `(m+1)` threshold values. Herein, each threshold value is designated by `T.sub.i `, wherein `i` is a variable representing an integer which belongs to an integral range between `1` and `m`. The (m+1) threshold values are designated by `T.sub.0 ` to `T.sub.m ` respectively, which meet a relationship of T.sub.0 &lt; . . . &lt;T.sub.i-1 &lt;T.sub.i &lt; . . . &lt;T.sub.m. So, specific data D.sub.jk which meet an inequality of T.sub.i-1 &lt;D.sub.jk .ltoreq.T.sub.i are allocated to a group `i`, i.e., "No. i" group within the m groups, for example.
A numeral 3 designates a count section which counts a number of the data D.sub.jk, which are allocated to each group by the allocation section 2, i.e., a number of dots `Si` for each group. A numeral 4 designates a residual-heat-value prediction section which uses one line of data to perform calculations in accordance with a formula (1). Herein, the residual-heat-value prediction section 4 calculates an amount of heat (i.e., residual-heat value) `Q`, which remains in the thermal head after printing of one line, by prediction. EQU Q=f(.SIGMA..sub.i=1.sup.m Ai, Si, T, k) (1)
In the above formula (1), `T` designates a print period for one line of printing; `Ai` designates a count number which is assigned to each group; `f` designates a predetermined function using parameters Ai, Si, T and k. The function and parameters are set in advance based on usage environment regarding characteristics of the thermal head, printer and ink.
A numeral 5 designates a print-data calculation section which calculates print data d.sub.jk in accordance with a formula (2), wherein the print data d.sub.jk are data actually transferred to the thermal head for the printing. EQU d.sub.jk =F(Q, D.sub.jk) (2)
In the formula (2), `F` designates a function which is set in advance in accordance with usage environment. Next, a numeral 6 designates a print-data transfer section which transfers one line of print data `d.sub.k ` calculated by the print-data calculation section 5, to the thermal head, wherein `d.sub.k ` represents one line of data for a line k. A numeral 7 designates a thermal-head drive section which drives the thermal head based on the print data transferred thereto from the print-data transfer section 6. Next, operation of the line-type thermoelectric printer, which is constructed as shown by FIG. 3, will be described with reference to a flowchart of FIG. 4.
When the printer inputs one line of data D.sub.11, D.sub.21, . . . , D.sub.n1 by the data-input section 1, the printer starts to execute procedures of the flowchart of FIG. 4. At first, the printer proceeds to steps S1 and S2, wherein initial setting is performed. Then, the printer proceeds to step S3 in which a decision is made as to whether a last line of printing has not been completed yet. In the current situation, however, the printer proceeds with process regarding a first line of printing (or printing of a line k where k=1). So, result of the decision turns to "YES". Thus, the printer proceeds with following procedures.
Steps S4 to S11 are provided to mainly perform two processes, i.e., an allocation process, which allocates the data D.sub.11, D.sub.21, . . . , D.sub.n1 to the m groups respectively, and a count process which counts a number of dots `Si` for each group. After completion of these processes, if result of a decision made by step S5 turns to `NO`, the calculations using the aforementioned formulae (1) and (2) are performed in steps S12 and S13. Thus, print data d.sub.11, d.sub.21, . . . , d.sub.n1 are calculated, so that printing is performed based on those print data in step S14.
Thereafter, the aforementioned processes are repeated with respect to a second line, a third line and other lines in turn. However, even if the line which is subjected to the processes is changed, the number of dots Si is not reset but is sequentially accumulated. Thus, it is possible to perform heat control of the thermal head with prediction of heat-accumulated state. After the processes are completed with respect to a line `n` so that result of a decision made by step S3 turns to `NO`, the printer ends the procedures of the flowchart.
By the way, the deviation of density in the line-type thermoelectric printer is varied in a variety of print patterns. In the line-type thermoelectric printer, there should occur a "tailing phenomenon" in which a density of a dot, which is printed following a dot printed in a high density, should become higher than a density expected. However, the line-type thermoelectric printer performs same heat control if the residual-heat value Q, calculated by the aforementioned formula (1), remains the same, regardless of print patterns. So, the conventional printer suffers from drawbacks that the printer cannot eliminate the trailing phenomenon and deviation of density which occur or alter responsive to the print pattern.
Meanwhile, a dye sublimation printer is provided to obtain a high print quality which is almost equivalent to the quality of photograph and is designed to differ printing energy with respect to each level of gradation (or each level of density). So, this printer is delicately affected by heat accumulation of the head thereof. Thus, it is demanded to provide a method or a device by which the printing energy is adequately controlled based on a heat-accumulated state which is predicted with consideration of the print pattern.
The data-input section 1 of the line-type thermoelectric printer shown in FIG. 3 is configured by a RAM having one page of storage (i.e., a page buffer). Therefore, this printer is designed to avoid a situation where a line of data to be processed are not inputted thereto from the computer. Recently, RAMs are made in a relatively low price. However, a high price should be required for a page buffer having a large capacity which is capable of storing one page of image data, for example.
If the printer does not provide such a large-capacity page buffer, there should occur a situation where a line of data to be processed are inputted thereto from a computer if the computer performs data transfer at a low speed. In such a situation, the printer should break printing operation during an unpredictable time before printing of a next line. If the printer re-starts the printing operation after the breaking, the residual-heat value Q of the thermal head is lowered responsive to a break time. However, the printer does not consider the break time. So, the residual-heat value Q calculated should be identical to the residual-heat value which is calculated under the condition where the printing operation smoothly continues without a break. In other words, the residual-heat value calculated should differ from a residual-heat value representing an amount of residual heat actually owned by the thermal head. And, a difference between them should increase responsive to the break time. As a result, the printer should provide an expensive page buffer in order to avoid occurrence of the deviation of density (or deviation of color). However, provision of the page buffer does not perfectly avoid occurrence of the deviation of density because of the reasons described before.